The present invention relates to a method of producing hollow shaped articles, such as bottles, vials, cans, other types of containers or the like, in accordance with the blow molding technique. More particularly, the invention relates to improvements in a multi-stage blow molding method according to which a plasticized parison consisting of synthetic thermoplastic material is first converted into a hollow preform or blank and the blank is thereupon converted into a hollow article of desired size and shape.
In accordance with a presently known method which involves so-called biaxial stretching of synthetic thermoplastic material, a tubular parison or a parison consisting of one or more strips of plasticized synthetic thermoplastic material is introduced into a first open-and-shut mold and is converted therein into a hollow blank or preform which resembles but is not identical with the final product. As a rule, the blank is slightly smaller than the shaped article. The blank is thereupon transferred into a second open-and-shut mold and is converted therein into a final product, i.e., a hollow shaped article having a predetermined size and shape. Reference may be had to German printed publication No. 2,161,066 or 2,164,526. The two molds are mounted on a common carrier or platen and open and close simultaneously so as to respectively allow for introduction of a parison into the first mold or the introduction of a blank into the second mold, or for removal of a blank from the first mold and removal of a shaped article from the second mold. When the extruding machine produces a parison at an extruding station, the platen with the two molds is moved in a first direction so that the open first mold receives the parison at the extruding station. The molds are then closed and the parison is separated from the leader of the plasticized material which issues from the nozzle of the extrusion machine. The platen is then moved in the opposite direction to move the first mold to a first blowing station where the parison is expanded to form a blank by resorting to a first blowing mandrel which descends into the upper end of the parison in the closed first mold. The molds are thereupon opened again and the platen is moved in the first direction to return the first mold to the extruding station where the next-following parison is ready to be separated from the extrusion machine. At the same time, the open second mold receives the blank at the first blowing station and thereupon closes with the first mold so that the first mold confines the next parison and the second mold confines the blank. The blowing mandrel at the first blowing station is withdrawn from the blank in the second mold and the platen moves in the opposite direction so as to return the first mold to the first blowing station and to locate the second mold at a second blowing station. The first mandrel penetrates into the parison in the first mold at the first blowing station and a second blowing mandrel penetrates into the blank in the second mold at the second blowing station. The mandrels then admit a suitable gaseous blowing medium so that the parison at the first blowing station is converted into a blank and the blank at the second blowing station is converted into a shaped article. The first blowing mandrel supports the blanks at the first blowing station while the platen moves in the first direction, and the shaped articles are segregated from the second mandrel at the second blowing station while the platen moves in such first direction. The opening of molds takes place when the shaped article in the second mold (at the second blowing station) is sufficiently rigid to be capable of withstanding treatment during and subsequent to transport from the second blowing station. The same cycle is then repeated again and again so that the first mandrel expands successive parisons at the first blowing station and the second mandrel expands successive blanks at the second blowing station.
The just described procedure exhibits a number of serious drawbacks. Thus, the withdrawal of first mandrel from the blank or preform at the first blowing station can result in damage to or substantial deformation of the neck portion of a blank which resembles a bottle, can, vial or a similar container. The deformation becomes even more pronounced when the neck of the blank receives the tip of the second blowing mandrel at the second blowing station. It is to be noted that the material of the blank at the first blowing station must remain in a plasticized state since the blank must undergo further deformation upon arrival at the second blowing station. The situation is further aggravated if the shaped article is to be provided with an externall or internally threaded or ribbed neck portion and/or if the neck portion is to be formed with a smooth and plane top end face in order to insure satisfactory sealing by means of a cap or the like. The wall thickness of the neck portion of a bottle normally exceeds the wall thickness of the main body portion of the bottle so that the hardening of neck portion proceeds rather slowly. This means that the first blowing mandrel is highly likely to deform the neck portion of a blank at the first blowing station, as well as that the second blowing mandrel is even more likely to again deform the neck portion during penetration into the blank at the second blowing station. It has been found that the just discussed method cannot be resorted to for the making of hollow shaped articles wherein the portions which are contacted by several discrete blowing mandrels have an accurately reproducible size and/or shape. The just discussed method is further totally unsuited for the making of bottles or analogous containers which are provided with internal threads, grooves, ridges or the like because such internal projections disappear or are deformed beyond repair during extraction of the first blowing mandrel and/or during insertion of the second blowing mandrel.